Comment by jeroenhd

I too would much prefer it if Google were to open-source their messaging app and provide a full RCS API to Android users, but that doesn't change the fact that both the protocol and the underlying OS are open, and others can implement them. Google isn't the Linux Foundation, you shouldn't rely on them to release everything open source and ready to use.

The AOSP messaging app is barebones to say the least, and has been abandoned since the day Google announced Google Messages and its predecessors. Back when that app was still relevant, RCS practically didn't exist.

RCS is not as simple as SMS. You need access to the SIP/RTSP/RTP layer to do video calling, for instance. RCS registration is tied directly to IMS registration, it was never designed to have multiple apps use the protocol at the same time; it does support multi-device setups, as long as all partipants run compatible software. Break that, and you break basic message reception.

Skipping features that go beyond basic messaging entirely, you could probably have some limited RCS support if Google were to take care of all of the internals. You would still need someone to do regular maintenance, like updating protocols to support things like gRPC transports (which landed this month), taking care of the end-to-end encryption, and whatever else comes up in the future. End-to-end encryption in RCS has been in the spec for maybe a year and a half, Android 14 and lower would not be able to use it at all, and Android 15 would miss out on Apple-compatibility as well. Not sure how you would deal with the payment exchange API in such a system, that would need some carrier/bank-run verification scheme.

AOSP's RCS implementation (https://source.android.com/docs/core/connect/ims-single-regi...) was never used by Android's messenger, it only provides very basic functionality: it exists because it has to for basic IMS operations to work, but Google never really bothered with RCS until very recently, long after they stopped maintaining their SMS app.

One could ask the same of other messengers: why can I not access Signal's message database? Why can I not enumerate emails, or query for browsers' search histories? All are built on open technologies, after all.